Most bacteria are harmless. In fact, many are beneficial to man and to other mammalian animals. However, certain bacteria are able to grow and to spread in human and mammalian tissues. One class of such virulent organisms of particular interest is the intracellular pathogens. This category of virulent pathogens multiplies within the cells of the infected host organism rather than extracellularly.
The broad classification of intracellular pathogens includes organisms that are major causes of morbidity and fatality world-wide. For example, intracellular pathogens are responsible for the estimated 10,000,000 new cases of tuberculosis per year in the world (approximately 25,000 per year in the United States), the approximately 3,000,000 deaths per year from tuberculosis, and the estimated 12,000,000 cases of leprosy. They are also responsible for the estimated 10,000,000 cases of American trypanosomiasis (Chagas disease). Additionally, intracellular pathogens also cause other important diseases including cutaneous and visceral leishmaniasis, listeriosis, toxoplasmosis, histoplasmosis, trachoma, psittacosis, Q-fever, and Legionellosis including Legionnaires' disease. Few vaccines are available against such intracellular pathogen engendered diseases. The only widely used vaccine is BCG vaccine against tuberculosis. BCG vaccine is a live bacterial vaccine of questionable efficacy used primarily in Europe.
Leoionella pneumophilathe causative agent of Legionnaires' disease is a particularly troublesome intracellular pathogen because it multiplies intracellularly in monocytes and macrophages, the very cells of the immune system intended to defend against it. Thus, L. pneumophila is in a class of intracellular pathogens which have actually circumvented the normal immune defenses by taking the cells of the immune system designed to kill bacteria and using them as host cells. Accordingly, L. pneumophila is particularly well suited to demonstrating the principles of the present invention.
Those skilled in the art will appreciate that the following exemplary discussion of L. pneumophila is in no way intended to limit the scope of the present invention to the treatment of Legionellosis or Legionnaires' disease alone and that the treatment of other intracellular pathogens is within the scope of the present invention.
The clinical investigation of Legionnaires' disease began in Jul. of 1976 when 221 people, mostly American Legionnaires' were struck with the disease in Philadelphia. Ultimately, 34 people lost their lives. Newly recognized, it was subsequently determined that Legionnaires' disease was not new and that it occurs world-wide in epidemic and endemic forms. It is believed to be a major cause of pneumonia in the United States and is responsible for a substantial proportion of fatal nosocomial pneumonia. Though affecting all age groups, it particularly affects those over 30 years of age, and targets men more often than women. The disease also demonstrates a predilection for individuals who smoke or abuse alcohol, travelers, construction workers, immunocompromised individuals and transplant patients. The case-fatality rate ranges between approximately 15%-20%.
L. pneumophila is an aerobic gram-negative bacterium with fastidious growth requirements. The organism is believed to be spread to humans and other mammalian hosts from contaminated sources by the airborne route. Ubiquitous in the environment, L. pneumophila has been isolated from water, soil, mud, and the cooling towers of air conditioning systems. It has also been associated with whirlpools and shower heads which can generate aerosols carrying the bacteria.
Human patients infected with this facultative intracellular bacterial pathogen develop both humoral and cell-mediated immune responses. Humoral immune responses appear to play only a secondary role in host defense against L. pneumophila because antibody does not promote killing of the bacterial pathogen by complement. Moreover, it promotes only modest killing by phagocytes and does not inhibit the intracellular multiplication of L. pneumophila in monocytes. Conversely, cell-mediated immune responses appear to play a primary role in host defenses against L. pneumophila because activated monocytes and alveolar macrophages inhibit its intracellular multiplication.
An excellent mammalian model for the study of Legionnaires' disease is the guinea pig which shares with humans a susceptibility to lung infection with L. pneumophila. After an incubation period of several days, guinea pigs exposed to aerosols containing L. pneumophila develop a pneumonic illness characterized by fever, weight loss, and labored respirations that sometimes culminates in death. This infectious syndrome closely parallels the clinical and pathological syndrome of Legionnaires' disease in humans. As with human patients, when exposed to a sublethal dose of L. pneumophila, guinea pigs also develop humoral and cell-mediated immune responses. Such guinea pigs also develop protective immunity to subsequent challenge with lethal doses of L. pneumophila. L. pneumophila is highly virulent for guinea pigs and is lethal to this animal by both the intraperitoneal and aerosol routes of inoculation.
Accordingly, for purposes of explanation only and not for purposes of limitation, the present invention will be discussed in the exemplary context of guinea pigs as the mammalian host. Those skilled in the art will appreciate that the present invention may be practiced with other mammalian hosts including humans.
It should be noted that L. pneumophila and other Legionella organisms are the causative agents for a variety of syndromes collectively referred to as "Legionellosis." In addition to Legionnaires' disease, Legionellosis includes Pontiac fever, Endocarditis and neurologic symptoms. Currently, no vaccine against Legionella engendered diseases exists. Moreover, as noted above, few vaccines exist against diseases caused by intracellular pathogens in general.
Compounding the search for effective vaccines against intracellular pathogens including L. pneumophila is the fact that these virulent pathogens are normally sequestered within cells of the host organism and thus are not readily detectable by the host's immune system. Moreover, with some intracellular pathogens, including L. pneumophila, antibodies produced against bacterial surface components actually induce uptake of the pathogens into the intracellular milieu they require to multiply. Accordingly, conventional vaccines which are traditionally directed against such bacterial cell surface antigens may actually stimulate the proliferation of these infectious organisms rather than resolving the infection. Thus, conventional vaccines directed against bacterial surface components may be contraindicated in the case of intracellular pathogens. Such conventional vaccines might be a particular problem for persons with depressed cell-mediated immunity, the form of immunity that primarily defends against infection with intracellular pathogens, because such persons would have diminished capacity to fight the more rapidly proliferating infection. Such persons include immunocompromised or immunosuppressed patients, such as organ transplant recipients or individuals infected with HIV, all of whom are particularly susceptible to infection with intracellular pathogens.
Accordingly, it is a principal object of the present invention to provide vaccines and methods for their use in mounting an effective immune response against intracellular pathogens such as L. pneumophila.
It is an additional object of the present invention to provide vaccines exhibiting reduced toxicity relative to traditional whole bacterial vaccines.
It is a further additional object of the present invention to provide vaccines and methods for their use which will not induce phagocytosis or cellular uptake of intracellular pathogens.
It is a still further additional object of the present invention to provide vaccines and methods for their use which will enable a vaccinated mammalian host to detect pathogens sequestered in host cells thereby allowing the host's immune system to kill or inhibit multiplication of the infecting pathogen.